The present invention relates to a process for preparing polyethylenes having a broad molecular weight distribution. More particularly, it is concerned with a continuous process for preparing polyethylene of varying polydispersities by combining an aluminoxane with a chromium compound and varying the ratio of these metal components.
In general, polyolefins used for obtaining molded or formed articles such as bottles, cable conduits and ultrathin films are required to fully withstand molding or forming conditions in plastisized state and be formed into desired shapes easily. This requirement may be satisfied by the use of a polyolefin having an increased melt index (a lowered average molecular weight). Such a polyolefin, however, can only afford a product inferior in strength, e.g. impact resistance and tensile strength. On the other hand, a polyolefin having a low melt index affords a product superior in strength, which product, however, is inferior in moldability. It is known that this problem can be solved by using a polyolefin having a broad molecular weight distribution.
Moreover, physical properties required for polyethylenes have been diversified recently, and also from the standpoint of resources saving there is a tendency to using polymer resin in an amount as small as possible in a range not impairing physical properties. For example, as to bottles and films, there is now a tendency to make them as thin as possible while maintaining their strength. A polyethylene which even in a small amount exhibits good processability and high impact strength, tensile strength and resistance to environmental stress cracking is keenly desired. Ziegler-Natta and chromium based systems comprise the two major classes of olefin polymerization catalysts. Aluminoxanes, especially methylaluminoxane, have recently found wide application, in conjunction with Group IV and V metallocenes, as components of Ziegler-type olefin polymerization catalysts. (See, for example, numerous papers by W. Kaminsky and J. C. W. Chien).
In contrast, the use of aluminoxanes in conjunction with chromium catalysts is limited. Catalysts produced from aluminoxanes and chromium salts, usually alkanoates, and an electron donating ligand such as hydrocarbyl isonitriles, amines, or ethers (U.S. Pat. No. 4,668,838 to Briggs), or carbon dioxide (U.S. Pat. No. 4,777,315 to Levine) have been reported as ethylene trimerization catalysts. Some polyethylene is also produced.
In the late 1960's Manyik et al. disclosed polymerization catalysts and processes which produced polyethylenes having a broad molecular weight distribution. U.S. Pat. Nos. 3,231,550 and 3,242,099 disclose polymerization of mono-unsaturated alpha-olefins to produce solid high molecular weight polymers by contacting them with a catalyst complex comprised of (1) poly(hydrocarbylaluminum oxides), i.e. an aluminoxane, and (2) a transition metal compound of the metals of Groups IVA, VA, and VIA; chromium is exemplified. Manyik states that,
"The mole ratio of the transition metal in the transition metal compound to the aluminum in the poly(hydrocarbylaluminum oxide) can be varied from 1:30 to about 1:800 but is preferably from about 1:40 to 1:200. By varying the ratios of the components used to produce the catalyst complex and the components employed and by varying the temperature, pressure, and time of reaction, the properties of the polyolefin can be varied." PA1 a) continuously reacting an alpha-olefin with a catalytically effective combination of a chromium compound and an aluminoxane at a first aluminoxane to chromium ratio to produce a polyalpha-olefin having a first molecular weight distribution; and PA1 b) changing the aluminoxane to chromium ratio by at least five percent thereby giving a second aluminoxane to chromium ratio to produce a polyalpha-olefin having a second molecular weight distribution. PA1 (a) combining a catalytically effective amount of a chromium compound and an aluminoxane in a hydrocarbon solvent, to produce a catalyst; PA1 (b) mixing said catalyst with an alpha-olefin at a temperature of between 75 to 110.degree. C. and at a olefin pressure of from 50-550 psi for an average residence time of from one to five hours; and PA1 (c) varying the ratio of said aluminoxane to said chromium compound so that the molecular weight distribution of the resulting polyethylene is modified over time.
A later patent to Manyik, U.S. Pat. No. 3,347,840, discloses an improved polymerization process for ethylene polymerization. As before, this process uses a catalyst complex of poly(hydrocarbylaluminum oxides) and a transition metal compound of the metals of Groups IV-A, V-A and VI-A. Here however, conversion of ethylene to 1-hexene is retarded drastically by the addition of small amounts of 1,3-dienes, such as butadiene. This improvement was in response to one of the disadvantages of the previous processes, specifically, the conversion of appreciable amounts of ethylene to butene-1 or hexene-1.
More recently, Canadian Patent Application No. 2,000,567 to Tajima et al. disclosed a composite catalyst consisting of a chromium compound, an aluminoxane, and an aluminum alkoxide. This catalyst produces polymers with improved rheological properties. This patent, in comparative examples 1, 2 and 3 on page 18 purports to show combinations of aluminoxanes and chromium catalysts, without aluminum alkoxides. These comparative examples, however were apparently somehow mislabeled, and the results are inscrutable.
U.S. Pat. No. 5,013,802, also to Tajima et al, discloses a process for the preparation of polyethylene with a broad molecular weight distribution. The process uses two catalysts in series. The first catalyst is a calcined chromium-oxide supported catalyst combined with modified organoaluminum compound which is produced by hydrolysis of a trialkylaluminum. The second catalyst consists of an organomagnesium compound and titanium. In the first stage of the polymerization, a high molecular weight polymer is produced when the chromium-oxide supported solid catalyst and modified organoaluminum compound is used. In the second stage, relatively low molecular weight polymer results when a solid catalyst with at least magnesium, titanium, and an aluminum compound is used. When this two stage highly productive process is employed, a well balanced ethylene polymer or copolymer with large melt tension, good processability and high ESCR is produced.
U.S. Pat. No. 4,701,432 to Welburn teaches varying the molecular weight distribution by varying the molar ratios of metallocene to transition metal. The catalyst system described in this invention consists of a catalyst comprising a metallocene of Group IV-B or V-B metal and at least one non-metallocene of Group IV-B, V-B or V-I transition metal. A supported co-catalyst is also taught to be used in this invention comprising aluminoxane and organometallic compound of Group I-A, II-A, II-B and III-A.
U.S. Pat. Nos. 4,791,180 and 4,752,597 both purport to disclose olefin polymerization catalysts comprising the reaction product of an aluminoxane with a metallocene complex of among others, Group VIb metals, although metallocenes of Group VIb are neither exemplified nor discussed.
As described above, many approaches have been tried to produce polymers with broad molecular weight distributions. Still, there is a need for simple process where the molecular weight distribution of the polymer can be readily adjusted by varying polymerization parameters or reactants.